1. Field of the Invention
The present invention relates to preparation of semiconductor materials, and, more particularly, to the preparation of mercury cadmium telluride and related materials.
2. Description of the Related Art
Alloys of mercury telluride and cadmium telluride, generically denoted Hg.sub.1-x Cd.sub.x Te, are extensively employed as photosensitive semiconductors for infrared radiation. Indeed, Hg.sub.0.8 Cd.sub.0.2 Te has a bandgap of about 0.1 eV which corresponds to a photon wavelength of 12 .mu.m and Hg.sub.0.73 Cd.sub.0.27 Te a bandgap of about 0.24 eV corresponding to a photon wavelength of 5 .mu.m; and these two wavelengths are in the two atmospheric windows of greatest interest for infrared detectors.
Reproducible preparation of Hg.sub.1-x Cd.sub.x Te with low defect densities has proved to be difficult. Typical preparation methods include a recrystallization at high temperatures followed by a low temperature anneal of the ingot in a saturated mercury atmosphere; the recrystallization at high temperatures generates excess tellurium and metal vacancies, and the low temperature anneal reduces the concentration of metal vacancies. FIG. 1a illustrates this preparation: point "A" is the recrystallization, the vertical down arrow from "A" represents the cooling of the ingot, the vertical up arrow represents heating the ingot up to anneal temperature, and the horizontal arrow represents the low temperature anneal resulting in the composition represented by point "B".
The method of U.S. Pat. No. 4,481,044 (Schaake and Tregilgas) for Hg.sub.1-x Cd.sub.x Te with x.apprxeq.0.2 includes recrystallization at a temperature in the range of 650.degree. to 670.degree. C. followed by a high temperature anneal of the ingot at about 600.degree. C. in a mercury atmosphere to reduce the excess tellurium and attendant dislocations. Subsequently, slices are annealed at a low temperature about 270.degree. C. in a mercury atmosphere for extended times to reduce the concentration of metal vacancies; this processing yields an n-type skin free of excess tellurium and a p-type core of condensed metal vacancies, precipitated tellurium, and gettered impurities. The thickness of the n-type skin increases with increasing anneal times, and with long anneal times the p-type core is annihilated. FIG. 1b illustrates this method.
Devices are fabricated in the n-type skin, which for bulk recrystallized Hg.sub.1-x Cd.sub.x Te may range from about 100 .mu.m to 350 .mu.m or more in thickness. Usually the annealed surface of a the Hg.sub.1-x Cd.sub.x Te is removed by polishing and etching to prepare it for device fabrication. The amount of material removed from the surface of the Hg.sub.1-x Cd.sub.x Te depends on the type of devices being fabricated and their sensitivity to surface damage from handling, as well as, variations in material properties which can come from in-diffusion of surface contamination impurities. Usually more than 25 .mu.m of surface material is removed for device preparation. For example, in U.S. Pat. No. 4,686,373 (Tew and Lewis) a slice of Hg.sub.1-x Cd.sub.x Te is first lapped and polished and then glued to a silicon chip containing processing circuitry. Next, the Hg.sub.1-x Cd.sub.x Te is thinned down to a thickness of about 12 .mu.m, and an insulator plus transparent gate are deposited on the exposed surface to form the infrared detector. In this case, the portion of the original Hg.sub.1-x Cd.sub.x Te slice that is used for the infrared detector was several .mu.m below the original surface of the slice.
However, the known preparation methods for Hg.sub.1-x Cd.sub.x Te do not provide slices with uniform electrical properties as a function of depth in regions where infrared detectors are usually built.